- the future (concrete) Galois `Field` implementation
shall receive the name `GF2` (as per common math notation)
=> name conflict with the current `GF2` class
implementing the elements of the future Galois `Field`
=> rename the current `GF2` class into `GF2Element`
- because `GF2Element` is a bit tedius to type for the end user,
we introduce a `gf2` alias in line with the naming convention
for the built-in data types (e.g., `int` or `float`)
that are also used as elements of (other) `Field`s
=> name conflict with the current `lalib.elements.gf2` module
=> rename the module into `lalib.elements.galois`
- adjust the docstrings to refer to "the `gf2` type"
- adjust the top-level imports and tests
- add `GF2` class in the `lalib.elements` sub-package
implementing a typical Galois field with two elements
- the singleton objects `one` and `zero` are the concrete
instances of the `GF2` type for the end users
- besides the typical Galois arithmetic, `one` and `zero`
behave like the built-in numbers `1` and `0`
and implement the `numbers.Rational` interface
- add exhaustive docstrings with usage examples
- add (unit) test cases with 100% coverage
